The present invention relates to an electronic fuel injection system for an internal combustion engine, provided with power correction means for increasing a quantity of fuel injection when the engine is in a highly loaded state.
Referring to FIGS. 1 and 2, description will be made as to a typical example of a system for increasing a quantity of fuel injection when an internal combustion engine mounted on a car or the like comes into a highly loaded state, that is, a so-called power correction system, in the conventional electronic fuel injection apparatus in the internal combustion engine. First, a basic pulse width T.sub.p of a valve opening pulse for opening a fuel injection valve is calculated through the following expression (1) on the basis of a revolutional speed N (r.p.m.) of the engine and a quantity Q.sub.a of an air flow sucked into the engine. EQU T.sub.p =K(Q.sub.a /N) (1)
(K: a constant)
Next, a correction factor K.sub.AF ' for a ratio of air-fuel mixture (hereinafter simply referred to as "air-fuel ratio") corresponding to the revolutional speed N of the engine, and the calculated basic pulse width T.sub.p is retrieved from a map, the correction factor K.sub.AF ' being used for compensating the characteristics of the injection valve, an air flow meter, or the like. A valve opening pulse width (that is, a period of fuel injection) T.sub.i actually applied to the fuel injection valve is obtained on the basis of the basic pulse width T.sub.p and the thus obtained correction factor K.sub.AF ' through the following expression (2). EQU T.sub.i =T.sub.p (1+K.sub.AF ') (2)
Assume now that the revolutional speed N of the engine is kept constant. The basic pulse width T.sub.p is increased in response to the increase in engine load before a predetermined value T.sub.p4 is reached, with the correction factor K.sub.AF ' kept zero. Thereafter, the value of the correction factor K.sub.AF ' is increased stepwise to decrease the air-fuel ratio to thereby gradually make the air-fuel mixture rich. That is, the value of the correction factor K.sub.AF ' is gradually increased in a transition region T.sub.p4 -T.sub.p5 before the basic pulse width T.sub.p reaches a threshold value T.sub.p5 of a highly loaded region, that is, a power correction region. Thereafter, that is when the basic pulse width T.sub.p comes into the power correction region, the correction factor K.sub.AF ' is kept at a substantially constant value. Thus, conventionally, when the basic pulse width T.sub.p comes into the power correction region, the injection pulse width is increased with a large correction factor K.sub.AF ' to increase the engine output.
However, the pulsation of suction air in a cylinder of an engine becomes apt to be transmitted to an air flow sensor disposed in the upstream of a throttle valve in a suction pass as the opening degree of the throttle valve is made larger, that is, as the basic pulse width T.sub.p is increased, and therefore the output signal of the air flow sensor representing the quantity of air flow Q.sub.a becomes apt to change or pulsate. As the quantity of air flow Q.sub.a pulsates, the basic pulse width T.sub.p obtained through the expression (1) also pulsates so as to cause the correction factor K.sub.AF ' to fluctuate. This fluctuation in correction factor K.sub.AF ' is violent in the transition region T.sub.p4 -T.sub.p5 where the correction factor K.sub.AF ' is increased stepwise as the basic pulse width T.sub.p is increased. Consequently, as shown in FIG. 2, in the case where the basic pulse width T.sub.p takes a value in the transition region T.sub.p4 -T.sub.p5, the change in correction factor K.sub. AF ' is large and therefore the degree of variation of the air-fuel ratio may exceed its target control value 0.4 to thereby change the revolutional speed of the engine to deteriorate the operation property of the engine and comfortable ride.
Further, the rate of fuel consumption becomes bad in the transition region T.sub.p4 -T.sub.p5 because the air-fuel ratio is made unnecessarily rich.